The present invention is related to nonlinear optical materials, methods of crystal growth, and devices employing such materials. More specifically, the present invention is related to nonlinear optical materials that satisfy the general formula Ba2xe2x88x92xMxB10O17, wherein M is a divalent metal ion; wherein x ranges from about 0 to 0.3.
Nonlinear optical (NLO) materials are unusual in that they affect the properties of light. A well-known example is the polarization of light by certain materials, such as when materials rotate the polarization vectors of absorbed light. If the effect on the polarization vector by the absorbed light is linear, then light emitted by the material has the same frequency as the absorbed light. NLO materials affect the polarization vector of the absorbed light in a nonlinear manner. As a result, the frequency of the light emitted by a nonlinear optical material is affected.
More specifically, when a beam of coherent light of a given frequency, such as produced by a laser, propagates through a properly oriented NLO crystal having non-zero components of the second order polarizability tensor, the crystal will generate light at a different frequency, thus extending the useful frequency range of the laser. Generation of this light can be ascribed to processes such as sum-frequency generation (SFG), difference-frequency generation (DFG) and optical parametric amplification (OPA). Devices using NLO crystals include, but are not limited to up and down frequency converters, optical parametric oscillators, optical rectifiers, and optical switches.
Frequency generation in NLO materials is an important effect. For example, two monochromatic electromagnetic waves with frequencies xcfx891 and xcfx892 propagating through a properly oriented NLO crystal can result in generation of light at a variety of frequencies. Mechanisms defining the frequency of light using these two separate frequencies are sum-frequency generation and difference-frequency generation. SFG is a process where light of frequency xcfx893 is generated as the sum of the two incident frequencies, xcfx893=xcfx891+xcfx892. In other words, SFG is useful for converting long wavelength light to shorter wavelength light (e.g. near infrared to visible, or visible to ultraviolet). A special case of sum-frequency generation is second-harmonic generation (SHG) where xcfx893=2xcfx891, which is satisfied when the incident frequencies are equal, xcfx891=xcfx892. DFG is a process where light of frequency xcfx894 is generated as the difference of the incident frequencies xcfx894=xcfx891xe2x88x92xcfx892. DFG is useful for converting shorter wavelength light to longer wavelength light (e.g. visible to infrared). A special case of DFG is when xcfx891=xcfx892, hence xcfx894=0, which is known as optical rectification. Optical parametric oscillation is also a form of DFG and is used to produce light at tunable frequencies.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (picometers/volt [pm /V]), birefringence (xcex94n, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radianxc2x7cm), temperature acceptance (Kxc2x7cm), walk-off (radian), temperature dependent change in refractive index (dn/dt), optical transparency range (nm), and the optical damage threshold (watts/cm2). Desirable NLO crystals should possess an optimum combination of the above properties as defined by the specific application.
Borate crystals form a large group of inorganic NLO materials used in laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Beta Barium Borate (BBO: xcex2-BaB2O4), lithium triborate (LBO: LiB3O5), and cesium lithium borate (CLBO: CsLi(B3O5)2) are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in high power applications. Select properties suitable for generation of laser light from the mid-infrared to the ultraviolet for these crystals are listed in Table 1.
BBO has a favorable non-linearity (about 2 pm/V), transparency between 2600 mn and 190 nm, significant birefringence (necessary for phase matching), and a high damage threshold (15 GW/cm2, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance that can limit conversion efficiencies. The crystal is relatively difficult to grow to large sizes and is somewhat hygroscopic.
LBO has good UV transparency (absorption edge xe2x96xa1 160 nm) and possesses a high damage threshold (25 GW/cm2, 0.1 ns, 1064 nm). However, it has insufficient intrinsic birefringence for phase matching to generate deep UV radiation. Furthermore, LBO melts incongruently and must be prepared by flux-assisted crystal growth methods. This limits production efficiency that leads to small crystals and higher production costs.
CLBO appears to be a very promising material for high-power production of UV light due to a combination of high nonlinearity and high damage threshold. The crystal can also be manufactured to relatively large dimensions. Unfortunately, the crystal is exceedingly hygroscopic and invariably sorbs water from the air; hence, extreme care must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
With so many intrinsic physical parameters to optimize, known optical frequency converters, at present, are applicable to specific applications. A major factor limiting the advancement of laser applications is the inability of conventional NLO devices to generate laser light at desired wavelengths, power levels, and beam qualities. Currently-available NLO materials are not able to meet specifications required by many applications due to a number of factors that include: small nonlinear coefficients, bulk absorption in energy regions of interest, poor optical clarity, low damage thresholds, instability under operation, environmental degradation, difficulty in device integration, and high fiscal costs of manufacture. In many cases, the fundamental limit of conventional NLO materials has been met, and as a result, they are not able to meet specifications required by many present and future applications. Related properties and shortcomings are discussed in Chemistry of Materials, 1:492-508 (1989), Keszler, Curr. Opinion in Solid State and Mater. Sci. 1, 204 (1996). Becker Adv. Mater. 10(13) p. 979-992 (1998), which are hereby incorporated by reference.
Because of the large number and diversity of present and projected applications, no single NLO material can be optimized for all uses. Thus far only a limited number of efficient NLO materials have been commercialized, thereby creating a bottleneck in the use of lasers in the advancement of many key technology areas. As a result, there is a continuing search for and development of new NLO materials.
It is an object of the present invention to produce and utilize nonlinear optical materials according to Formula 1:
Ba2xe2x88x92xMxB10O17xe2x80x83xe2x80x83Formula 1
wherein M is a divalent metal ion; wherein x ranges from 0 to 0.3. Preferred divalent metal ions M that are suitable for forming compounds satisfying Formula 1 can be independently selected from the metals Ba, Sr, and Pb. Examples of nonlinear optical materials satisfying Formula 1 include, but are not limited to, Ba2B10O17, Ba1.8Sr0.2B10O17, and Ba1.9Pb0.1B10O17.
One embodiment comprises forming a mixture comprising from about 10 to about 25 mole % of a source of barium, and from about 75 to about 90 mole % of boron oxide. The mixture is heated to a temperature and for a period of time sufficient to form the NLO material. For instance, the step of heating may comprise heating the mixture to a first temperature of at least 825xc2x0 K, preferably greater than about 850xc2x0 K, for the period of one or more hours. The mixture is then cooled. After cooling, the mixture is comminuted (ground to a fine powder, such as by grinding with a mortar and pestle), and then heated to a second temperature of at least 1000xc2x0 K, preferably greater than about 1000xc2x0 K, for a period of one or more hours, followed by cooling to room temperature. If M is Ba, then the source of M generally is barium carbonate or barium nitrate. If M is Sr, then the source of M generally is strontium carbonate or strontium nitrate. If M is Pb, then the source of M generally is lead (II) oxide, lead(II) carbonate, or lead (II) nitrate.
It is a further object of the present invention to provide methods for making nonlinear optical crystals that satisfies Formula 1. One embodiment uses a standard Czochralski crystal growth technique to form large single crystals. Other embodiments include other crystal growth methods such as top-seeded solution growth, flux growth, horizontal Bridgeman, vertical Bridgeman, zone-refining, hydrothermal, or other methods.
The nonlinear optical materials of the present invention can be combined with other materials to form useful compositions, as long as the other materials do not unduly compromise the nonlinear optical features. For instance, the nonlinear optical materials may be mixed with inert materials to form composites that still exhibit nonlinear optical properties. Ba2B10O17 crystal(s) may be suspended in an optical plastic resin to form a composite nonlinear optical device, as an example.
It is a further object of the present invention to provide devices that use or require nonlinear optical materials for operation. An example of such a device, without limitation, is a harmonic generating crystal. A harmonic generating crystal is a crystal that is used to produce harmonic frequency outputs of inputted laser light frequencies. Another example, without limitation, of such a device is an optical parametric oscillator (OPO). An OPO uses NLO materials to produce widely tunable coherent light. Optical devices according to the present invention comprise a light source, such as a laser, optically coupled to nonlinear optical materials that satisfies Formula 1. Naturally, the device could include additional components, such as, without limitation, photodetectors, photomultipliers, crystal mounts, lens and/or mirror systems, cooling systems a control and/or data acquisition computer, and the like.
The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its structure and its operation together with the additional objects and advantages thereof will best be understood from the following description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawings. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the applicable art or arts. If any other meaning is intended, the specification will specifically state that a special meaning is being applied to a word or phrase. Likewise, the use of the words xe2x80x9cfunctionxe2x80x9d or xe2x80x9cmeansxe2x80x9d in the Description of Preferred Embodiments is not intended to indicate a desire to invoke the special provision of 35 U.S.C. xc2xa7112, paragraph 6 to define the invention. To the contrary, if the provisions of 35 U.S.C. xc2xa7112, paragraph 6, are sought to be invoked to define the invention(s), the claims will specifically state the phrases xe2x80x9cmeans forxe2x80x9d or xe2x80x9cstep forxe2x80x9d and a function, without also reciting in such phrases any structure, material, or act in support of the function. Even when the claims recite a xe2x80x9cmeans forxe2x80x9d or xe2x80x9cstep forxe2x80x9d performing a function, if they also recite any structure, material or acts in support of that means of step, then the intention is not to invoke the provisions of 35 U.S.C. xc2xa7112, paragraph 6. Moreover, even if the provisions of 35 U.S.C. xc2xa7112, paragraph 6, are invoked to define the inventions, it is intended that the inventions not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function, along with any and all known or later-developed equivalent structures, materials or acts for performing the claimed function.